There are roughly two conventional methods for obtaining a medical x-ray image.
With the first conventional method, the person capturing an x-ray image (hereinafter referred to as an image-capturing person) stores in advance for each configuration of the x-ray system and for each examined body part to be imaged, an imaging condition under which an x-ray image of a body meeting a predetermined standard (a standard predetermined based on the height, the weight, the chest and hip thicknesses, the race, and the age, for example) (the body meeting the predetermined standard is hereinafter referred to as a standard body) has an appropriate radiographic effect, in memory included in the x-ray system. It should be noted that examples of parameters of the imaging condition include an x-ray tube voltage (hereinafter referred to as a tube voltage), an x-ray tube current (hereinafter referred to as a tube current), an imaging time, and an imaging distance. In the case of analog x-ray images, the x-ray image having an appropriate radiographic effect (also referred to as an appropriate x-ray image) refers to an image having a radiographic density of about 1.0 in the examined body part, for example. In the case of digital x-ray images, the appropriate x-ray image refers to an image having an exposure index (EI) value corresponding to the image quality of the analog x-ray image. Then, the image-capturing person compares the standard body and the examinee's body in terms of the thickness and the body build, for example, and when there is a difference in the thickness and/or the body build, modifies the predetermined standard-body imaging condition based on an imaging condition modification method such as an exponential method, and captures an image of the examinee's body using the modified imaging condition.
With the second conventional method, the image-capturing person determines, for each thickness of the examinee's body and each examined body part, the tube voltage and the tube current to be used, and irradiates the examined body part with x-rays. Then, while irradiating the examined body part with x-rays, the image-capturing person measures the dosage of x-rays passing through the examinee's body and captures an image of the examinee's body using an automatic exposure control (AEC) mechanism that blocks the x-ray irradiation when the dosage of the x-rays passing through the examinee's body reaches a predetermined x-ray dosage.
With the first image obtaining method, the imaging condition for the standard body is subjected to primary modification according to the thickness and the body build of the examinee's body. Furthermore, the imaging condition obtained by the primary modification is subjected to secondary modification in consideration of a motion of the examined body part and a magnification ratio that is dependent on the geometric position of the examined body part. As a result of such primary modification and secondary modification, an appropriate image is obtained. Therefore, for example, when the examined body part moves, image capturing is attempted under, as an imaging condition obtained by the secondary modification (hereinafter referred to as a secondarily modified condition), an imaging condition including a greater tube voltage, a greater tube current, and a shorter imaging time than the imaging condition obtained by the primary modification (hereinafter referred to as a primarily modified condition). Furthermore, for example, when the magnification ratio increases, image capturing is attempted using an x-ray tube having a small focal spot under, as the secondarily modified condition, an imaging condition including a tube current smaller than a tube current used for an x-ray tube having a large focal spot. However, at the time of the primary modification and the secondary modification, there are a large number of imaging conditions that give an analog image an equivalent radiographic density in the case of analog image capturing or a large number of imaging conditions that give an x-ray detector system an equivalent irradiation dose factor (hereinafter also referred to as EI-equivalent imaging conditions) in the case of digital image capturing. It should be noted that the imaging conditions that give an analog image an equivalent radiographic density and the EI-equivalent imaging conditions are hereinafter also collectively referred to as equivalent radiographic effect conditions.
As compared to the first image obtaining method, the second image obtaining method allows capturing an image of the examinee's body with a simple operation technique using the AEC mechanism. However, since the AEC mechanism does not take into consideration such a factor as a motion of the examined body part or the magnification ratio dependent on the geometric positional relationship of the examined body part, it may not be always possible to obtain an image appropriate for medical practice. Although the second image obtaining method makes it possible to obtain an appropriate image if a motion of the examined body part and the magnification ratio dependent on the geometric position of the examined body part are taken into consideration, experience and knowledge are required.
With the first image obtaining method, if a part of the parameters included in the imaging condition, that is, the tube voltage, the tube current, the imaging time, and the imaging distance, for example, is changed, it is also necessary to change the remaining parameters in order to obtain an equivalent radiographic effect condition, even if there is no change to the configuration of the x-ray system. A change of a parameter including a change of the tube voltage is particularly difficult. To address this difficulty, various imaging condition modification methods for efficiently changing (modifying) the imaging condition are publicly available.
Non-Patent Literature 1 (NPL 1) discloses some examples of such imaging condition modification methods.
Specifically, NPL 1 describes a method using an exponential function as one of the various imaging condition modification methods.